The present invention relates to radiofrequency ablation of tumors and the like and, in particular, to a multi-pronged probe device allowing for both bipolar and monopolar ablation from the prongs of the probe.
Ablation of tumors, such as liver (hepatic) tumors, uses heat or cold to kill tumor cells. In cryosurgical ablation, a probe is inserted during an open laparotomy and the tumor is frozen. In radiofrequency ablation (RFA), an electrode is inserted into the tumor and current passing from the electrode into the patient (to an electrical return typically being a large area plate on the patient's skin) destroys the tumor cells through resistive heating. While both methods are generally successful in treating tumor cells, RFA is advantageous in comparison to cryosurgical ablation because the treatment can be delivered percutaneously, without an incision, and thus with less trauma to the patient. In some cases, RFA is the only treatment the patient can withstand. Further, RFA can be completed while the patient is undergoing a CAT scan.
Typically, RFA is provided between an electrode probe and a contact plate provided on the skin of the patient in a process known as monopolar ablation. A simple RFA electrode for use in monopolar ablation is a conductive needle having an uninsulated tip placed within the tumor. The needle is energized with respect to a large area contact plate on the patient's skin by an oscillating electrical signal of approximately 460 kHz. Current flows radially from the tip of the needle and produces a spherical or ellipsoidal zone of heating (depending on the length of the exposed needle tip) and ultimately a lesion within a portion of the zone having sufficient temperature to kill the tumor cells. Treatment is therefore provided in a relatively confined area. For large tumors, therefore, multiple applications of monopolar ablation procedures are often required.
Another type of RFA electrode probe useful in monopolar ablation is an umbrella probe. The umbrella probe uses an umbrella-style electrode in which three or more electrode wires or prongs extend radially from the tip of the electrode shaft after it has been positioned in the tumor, thereby providing an increased electrode area as compared to the needle described above. The prongs are electrically connected and, in operation therefore, all of the prongs operate at the same voltage. As described above with respect to the needle probe, umbrella electrodes are typically energized with respect to a large area contact plate on the patient's skin. Current flowing radially from the tip of each of the prongs of the umbrella probe again produces a spherical or ellipsoidal zone of heating (depending on the length of the exposed needle tip). The combined zones of the prongs of the umbrella probe produces an enlarged zone of heating as compared to the needle probe. Again, however, the deposition of power is in a defined area based on the geometry of the probe, and it is often necessary to reposition the probe and provide multiple applications during treatment.
To further increase the effectiveness of treatment, two umbrellas placed locally around a tumor can also be used in an ablation process known as bipolar ablation. In bipolar ablation, current flows between the two umbrella probe electrodes, which are positioned under the patient's skin, rather than between an electrode and a contact plate. This current flow provides a deposition pattern which “focuses” the energy specifically on the tumor volume between the electrodes producing a lesion with higher heating and more current density between electrodes than would be obtained by a comparable number of monopolar umbrella electrodes operating individually. Bipolar operation therefore can provide more effective treatment of targeted tumors due to greater tissue heating with a single placement of electrodes, improving the speed and effectiveness of the procedure and making it easier to determine the treated volume over procedures where an individual electrode is moved multiple times. However, because two probes are required, bipolar ablation is largely limited to the treatment of only large tumors.
While a number of methods of both monopolar ablation and bipolar ablation are therefore known, each of these methods provides power dispersion to a tumor in a specific pattern based on the geometry of the probe or probes used in the ablation process. These processes, therefore, are fairly limited in that the output of the probes cannot be tailored specifically to a specific tumor. Prior art methods, therefore, provide limited ability to control the deposition of power within the tumor, or to provide the types of ablation most desirable for a given treatment situation without changing the geometry of the probe. Because of these limitations, RFA often fails to kill all of the tumor cells and, as a result, tumor recurrence rates of as high as 50% have been reported.